Re: [PATCH 5/6] KVM: arm64: Treat 32bit ID registers as RAZ/WI on 64bit-only system

Marc Zyngier
2022-08-23T18:42:52Z

On Wed, 17 Aug 2022 22:48:17 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
>
> One of the oddities of the architecture is that the AArch64 views of the
> AArch32 ID registers are UNKNOWN if AArch32 isn't implemented at any EL.
> Nonetheless, KVM exposes these registers to userspace for the sake of
> save/restore. It is possible that the UNKNOWN value could differ between
> systems, leading to a rejected write from userspace.
>
> Avoid the issue altogether by handling the AArch32 ID registers as
> RAZ/WI when on an AArch64-only system.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
> arch/arm64/kvm/sys_regs.c | 63 ++++++++++++++++++++++++++-------------
> 1 file changed, 43 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 9f06c85f26b8..5f6a633182c8 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1145,6 +1145,20 @@ static unsigned int id_visibility(const struct kvm_vcpu *vcpu,
> return 0;
> }
>
> +static unsigned int aa32_id_visibility(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *r)
> +{
> + /*
> + * AArch32 ID registers are UNKNOWN if AArch32 isn't implemented at any
> + * EL. Promote to RAZ/WI in order to guarantee consistency between
> + * systems.
> + */
> + if (!kvm_supports_32bit_el0())
> + return REG_RAZ | REG_USER_WI;

This is probably only a nit, but why does one visibility has a _USER_
tag while the other doesn't? In other word, what sysregs are WI from
userspace that aren't so from the guest?

Also, do we have any cases where RAZ and WI would be used
independently? My gut feeling is that RAZ implies WI in most (all?)
cases. If this assumption holds, shouldn't we simply rename REG_RAZ to
REG_RAZ_WI and be done with it?

Thanks,

M.

--
Without deviation from the norm, progress is not possible.